Process for producing liquefied hydrogen,helium and neon

ABSTRACT

A PROCESS FOR LIQUEFYING HYDROGEN, HELIUM AND NEON MORE EFFICIENTLY AND ECONOMICALLY THAN BY METHODS PREVIOUSLY PRACTICED, WHICH PROCESS INCLUDES THE STEPS OF COMPRESSING THE GAS (HYDROGEN, HELIUM OR NEON) TO A PRESSURE SUCH THAT, UPON ISOBARICALLY COOLING THE THUS COMPRESSED GAS, A TEMPERATURE ABOVE THE CRTITCAL TEMPERATURE OF THE GAS IS REACHED AT WHICH THE GAS CAN BE ISENTRIOPICALLY EXPANDED TO YIELD SUBSTANTIALLY A SINGLE LIQUID PHASE AT ATMOSPHERIC PRESSURE; THEN ISOBARICALLY COOLING THE GAS AT THIS PRESSURE TO SUCH TEMPERATURE; AND FINALLY ISENTROPICALLY EXPANDING THE GAS TO SUBSTANTIALLY ATMOSPHERIC PRESSURE THROUGH A WORK ENGINE.

L. GARWIN Oct. 5, 1971 PROCESS FOR PRODUGING LIQUEFIED HYDROGEN, HELIUMAND NEON 2 Sheets-Sheet l Filed April 25, 1969 TMOSPHEPS w m v m m Q 6ASEOUS A//TPOG/V L. GARWIN Oct. 5, 1971 PROCESS FOR PRODUGING LIQUEFIEDHYDROGEN, HELIUM AND NEON Filed April 25, 1969 2 Sheets-Shea?l 2TMOWHEQES INVENTOR. 50 M/)QAQw//v United States Patent Office 3,609,984Patented Oct. 5, 1971 U.S. Cl. 62-22 9 Claims ABSTRACT OF THE DISCLOSUREA process for liquefying hydrogen, helium and neon more efficiently andeconomically than by methods previously practiced, which processincludes the steps of compressing the gas (hydrogen, helium or neon) toa pressure such that, upon isobarically cooling the thus compressed gas,a temperature above the critical temperature of the gas is reached atwhich the gas can be isentropically expanded to yield substantially asingle liquid phase at atmospheric pressure; then isobarically coolingthe gas at this pressure to such temperature; and finally isentropicallyexpanding the gas to substantially atmospheric pressure through a workengine.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to a method for liquefying certain low molecular weight gases,and more particularly, to a method for more economically liquefyinghydrogen, helium and neon.

Brief description of the prior art Helium and hydrogen are presentlyoften industrially liquefied by initially cooling them by heat exchangeor other means, such as expansion engines, until they are brought belowtheir inversion temperature. They are then usually further cooled andliquefied by a Joule-Thomson expansion. The Joule-Thomson expansion is ahighly irreversible thermodynamic process, wasteful of process energyand therefore economically inefiicient. Its advantage is that theequipment used is simple and reliable. It is well recognized thatexpansion of the cooled gas in an engine which does outside work is amore efficient and economical process, but such method of liquefactionby expansion has not been used except in conjunction with a subsequentJoule-Thomson expansion due to the serious and frequently disastrousmechanical effects which occur when the expansion of the fluid throughthe turbine or engine produces both liquid and gaseous phases in thisequipment.

Quite recently it has been proposed to liquefy gases by a method whichentails expanding cooled compressed gas through a work engine underconditions where no liquid or gas phases concurrently exist. This methodis described in U.S. Pat. 3,383,873 to Becker. In the Becker process, agas is first compressed to a pressure above the critical pressure of thegas. This compressed gas is then isobarically cooled to a temperaturebelow the critical temperature. The cooled fiuid is then expandedthrough a turbine or other work engine under conditions wherein onlyliquelfied gas is present. 'Ihe engine-expanded cooled fluid is thenisobarically further cooled and this fluid is finally throttled toatmospheric pressure.

The process of Becker prescribes the isobaric cooling of the initiallycompressed gas to a temperature which is at, or below, the criticaltemperature of the gas in order to then isentropically expand the cooledgas through a work engine to produce only liquefied gas in the engine.Even then, however, with methane and the other types of gasescontemplated by Becker, unless cooling is carried out to a temperaturelower than the critical temperature, the isentropic expansion in thework engine Imust be carefully controlled to expand the cooled fluid toa pressure substantially above atmospheric pressure, since expansion tonear atmospheric pressure will result in the production of both liquidand undesirable quantities of gas in the engine. The above atmosphericpressure and temperature liquid produced by the method of Becker underthe controlled pressure condition by engine expansion is then removedfrom the engine and isobarically cooled further, followed by a finalexpansion through a throttle valve to atmospheric pressure. This finalexpansion results in the production of a smaller amount of gas.

BRIEF DESCRIPTION OF THE PRESENT INVENTION The present inventionprovides a process for improving the economy of liquefaction of certainlow molecular weight gases which are commonly characterized in havingthe one atmosphere point on the liquidus curve located not more than 20K. below the critical temperature. More specifically, the process of theinvention is broadly applicable ot the liquefaction of hydrogen, heliumand neon, and has marked specific advantages when used in theliquefaction of helium.

My discovery of the process of the present invention is based on a studyof the older published entropy-enthalpy characteristics of helium, afurther development of new data to establish the entropy-temperaturecharacteristics of helium at higher pressures (above 100 atmospheres)than those previously studied in this respect, and the perception of amore economical and efficient method for liquefying helium than thosewhich have previously been utilized in practice for this purpose, aswell as an improvement with respect to the Becker method. The method, inits broader aspects, was then perceived lto be applicable also to theliquefaction of hydrogen and neon, which have temperature-entropycharacteristics rendering them susceptible to the same basic principlesutilized in the helium liquefaction procedure.

Broadly described, the method of the present invention comprisescompressing the gas at a temperature above the critical temperature to apressure such that the gas may be isobarically cooled to a temperatureabove the critical temperature from which the gas may be isentropicallyexpanded through a work engine to produce, at atmospheric pressure, asingle liquid phase by conversion of substantially all of the gas toliquid. After compressing the gas in the manner described, it isisobarically cooled to a temperature such that the followingsubstantially isentropic expansion will result in the conversion ofsubstantially all the gas to a liquid at atmospheric pressure. Theisentropic expansion through a turbine or other work engine thencompletes the process.

The methods, as thus broadly described, is preferably practiced, in thecase of helium gas, in combination with certain other steps now utilizedin the production of helium from naturally occurring sources, where suchother steps yield a stream of helium at pressures of 20-200 atmospheresand temperatures of from about 65 K. to about 300 K., and mostpreferably from about 150 atmospheres to about 200 atmospheres. In suchpractice, the latter stream is used as a feed stream to the apparatusutilized in carrying out the broadly described steps characteristic ofthe present invention in its most general applicability.

It is an object of the present invention to provide a method foreconomically liquefying helium, hydrogen and neon by a final expansionthrough a Work engine.

Another object of the invention is to provide a method BRIEF DESCRIPTIONOF THE DRAWINGS FIG. 1 is a temperature-entropy diagram illustrating icertain of the thermodynamic properties of helium.

FIG. 2 is a temperature-entropy diagram illustrating certain of thethermodynamic properties of hydrogen.

FIG. 3 is a schematic flow sheet illustrating a preferred process of thepresent invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION It iscommon engineering knowledge that less work is required to liquefy a gasby compression at ordinary temperatures (assuming the gas is below itscrtical ternperature) than to liquefy it at ordinary pressures byremoval of the sensible heat and the latent heat of vaporization by heatexchange. To the extent that liquefaction of any gas can take place athigher temperatures, through pressurization, as contrasted with lowertemperatures by refrigeration, the process of liquefaction is moreefficient in terms of energy requirement. It is also well recognizedthat expansion of the gas through a work engine which does external workis a much more efiicient and economical method of cooling the gas,whether with or without liquefaction, than a Joule-Thomson expansionthrough an expansion valve. Where partial liquefaction occurs in thework engine, however, mechanical problems are encountered Which makethis approach to liquefaction of gases extremely difficult, and in manyinstances, infeasible.

By a study of the thermodynamic properties of helium and hydrogen, Ihave now perceived that these gaseous materials lend themselves in aunique way to liquefaction through optimum use of compression andsubsequent isentropic expansion through a work engine, with accompanyingminimization of the extent of cooling of the gases by refrigeration heatexchange required in the liquefaction process. The economy with whichthese gases can be liquefied is thus vastly improved over processeswhich effect the final cooling of the gases by a Joule-Thomson expansionthrough a valve. Moreover, the thermodynamic properties of neon aresufficiently analogous to those of helium and hydrogen that this gas canalso be liquefied by the process of the invention.

In referring to FIG. 1 of the drawings, it will be noted that theliquidus (saturated liquid) curve of the temperature-entropy diagram forhelium is relatively fiat, and that the one atmosphere liquid line islocated only about 1 K. below the critical temperature. As contrastedwith this thermodynamic property, the one atmospheric liquid line formethane is located about 79 K. below the critical temperature of thisgas in the temperature-entropy diagram, a general illustration of whichappears in the Becker patent to which reference has been made. Incomparing the temperature-entropy diagrams of methane and helium, itwill be perceived that, for different gases, as the one atmosphereliquid line becomes further removed from the critical temperature, a gascompressed above its critical pressure must be isobarically cooled to agreater degree (to a relatively lower temperature) before it can beisentropically expanded to yield saturated one atmosphere liquid. It isalso apparent that the critical pressure of helium (2.26 atmospheres) isrelatively low in comparison to that of methane (45.8 atmospheres).Thus, at a relatively low pressure, say, 25 atmospheres, the helium gasneed be cooled only to about 6 K., a temperature above its criticaltemperature, in order to then isentropically expand the gas to yield asaturated liquid at one atmosphere pressure. If allowance is made forsome irreversibility and energy losses in an expansion engine, thenperhaps the 25 atmosphere helium would need to be cooled to about thecritical temperature in order to avoid formation of gas in approximatingisentropic expansion by exhausting the gas through such an engine. Fromthe helium temperature-entropy diagram it can further be perceived thatif the helium is compressed to, say, 200 atmospheres, it then need onlybe cooled to about 10 K. in order to yield one atmosphere saturatedliquid upon isentropic expansion. As will be appreciated by thoseskilled in the cryogenics art, a large amount of work is required toabstract from a given quantity of helium gas sufficient heat to lowerits temperature from about 10 K. to about 6 K.; much less work isrequired to compress the gas from 25 atmospheres to about 200atmospheres than is required to remove the amount of heat necessary toachieve this drop in temperature. From my development of thetemperature-entropy curves for helium in the range of 100 to 400atmospheres, I have determined that there is no problem with theformation of solid helium during the compression of helium to severalhundred atmospheres, followed by the reduction of its temperature toaround from 6 K. to 12 K. Thus, helium lends itself Well to highcompression for the purpose of minimizing the amount of isobaric coolingsubsequently required in order to achieve, in accordance with thepresent invention, a final objective of isentropically expanding thethus compressed, cooled helium through a work engine in order to yieldsaturated liquid helium at atmospheric pressure.

Having perceived from the temperature-entropy diagram of FIG. l as Ihaw/e developed it in the above-100 atmosphere range for helium, thepossibility of highly compressing the helium in order to increase thetemperature from which it may be substantially isentropically expandedin a work engine to produce one atmosphere liquid, a comparison with theBecker method heretofore known further disclosed that this method ofliquefying helium possessed marked advantages over the Becker method.Moreover, the method of liquefying helium by initially compressing it toabove 100 atmospheres, followed by a minimal amount of isobaric cooling,provides a method of liquefaction which can be used quite well incombination with certain procedures now frequently employed in theproduction of helium which yield high pressure streams of relativelypure helium in the course of the process. For example, proceduresemployed by the U.S. Bureau of Mines, which produces a majority of thehigh purity helium produced in the United States, yield product streamsof helium at pressures of from about 150 atmospheres to about 200atmospheres, and at final purification temperatures of from about 65 K.to about K., before being brought back up to about 300 K. At theserelatively high pressures, the temperature can be economicallyisobarically reduced until the entropy of the gas allows isentropicexpansion of the gas to one atmosphere saturated liquid in an enginedoing wonk. Not only does the high pressure of the plant stream avoidthe necessity of lowering the temperature to, or below, the difficultyattainable critical temperature of helium, but the engine expansion tothe saturated one atmosphere liquid yields a percent liquid product atatmospheric pressure ready for storage. In helium liquefaction aspreviously practiced, final liquefaction of the compressed gas wasobtained by a Joule-Thomson expansion in which nok work was performedand only about one-third of the expandable gas liquefied.

A typical process ow diagram for the liquefaction of helium by theprocess of the present invention, and using a high pressure stream(about 2940 p.s.i.a. or about 200 atmospheres) of the type described isshown in FIG. 3. Reference to this figure in conjunction with an examplewill senve to illustrate the advantages afforded by the process of thepresent invention as compared to a helium liquefaction procedurecurrently in widespread usage.

Feed helium gas in the amount of 5000 standard cubic feet per hour, at apressure of 200 atm. and at ambient temperature, enters heat exchanger 1in which it is cooled to about 80 K. by countercurrent heat exchangewith refrigerant helium return gas and with refrigerant nitrogen, thelatter supplied by nitrogen liquefaction system .11, to which thenitrogen effluent from exchanger 1 returns. Thereupon, the feed heliumgas continues into heat exchanger 2, in which it is cooled byrefrigerant helium return gas to a temperature of about 40A K.Similarly, it proceeds to heat exchanger 3, in which its temperature islowered to about 24 K., then to heat exchanger 4, from which it leavesata temperature of about 18 K., and finally to heat exchanger 5, whichlowers its temperature to about 8 K. 'Ihe feed gas, still at essentially200 atm. (less a small pressure drop in exchangers 1 through isessentially isentropically expanded in expansion engine 6 to produce oneatmosphere liquid with little or no gas, in the amount of about 200liters per hour. The liquid is collected in receiver "I, from which itmay be withdrawn as needed.

The refrigeration required for cooling the helium feed stream isprovided by a conventional helium closed cycle refrigerator system,operating between approximately 18 atm. and 1 atm. The refrigeranthelium at 18 atm. from helium compressor 8 flows through exchangers 1and 2 and then splits, roughly 45 percent continuing through heatexchangers 3l and 4, while the remainder passes through high temperaturerefrigerant helium expander 9, in which it expands to l atm. and entersthe refrigerant helium return gas channel at the low temperature side ofexchanger 3, passing through exchangers 3, 2 and 1 in turn, and enteringthe suction side of refrigerant helium compressor 8 for compression andrecycling. The high pressure helium refrigerant stream leaving exchanger4 is expanded to 1 atm. in low temperature refrigerant helium expander10, flows back through exchanger 5 (in which it is the sole coolant forthe feed 'helium stream), thence through exchanger 4, after which itcombines with the eflluent from high temperature refrigerant heliumexpander 9, and the combined stream flows back to compressor 8 throughexchangers 3, 2 and 1, in turn.

The power heretofore typically required for the liquefaction of oneatmosphere helium at the rate of 5000 standard cubic feet per hour isabout 9 kw.h. per 100 s.c.f., or about 450 kw. By the method of thepresent invention, as illustrated in the foregoing example, the powerrequirements for liquefaction in a plant of the same capacity, startingwith 1 atmosphere gas, are only about 6.6 kw.h. per 100 s.c.f. If thegas is already available, because of prior processing, at a pressure of200 atm., as actually shown in the example, the liquefaction powerrequirements are reduced to about 6.0 kw.h. per 100 s.c.f. Theavailability of feed gas at 200 atm. instead of at Il atm. is of littlebenefit, power-wise, to the cost of liquefaction by conventional means.In some instances, the 200 atmosphere helium stream may be available ata temperature of 80 K. Where this is the case, the cooled, pressurizedhelium stream may be introduced to the system depicted in FIG. 3 in themanner shown by the dashed line. Thus, the heat exchanger 1 is partiallyby-passed, and there is a further decrease in the power requirement.

The similarity between the thermodynamic properties of hydrogen andhelium can be seen in comparing the temperature-entropy diagrams ofFIGS. l and 2. It will be noted that about 13 K. separates the criticaltemperature from the one atmosphere saturated liquid in the case ofhydrogen, thus making this material much more analogous to helium inthis respect than to methane to which reference has been previouslymade. Moreover, by compressing the hydrogen to about y600 atmospheres,it can then be isobarically cooled to a temperature of about 34 K.(above its critical temperature) without formation of a solid phase, andat this temperature and pressure it may be engine-expanded to yieldsaturated one atmosphere liquid.

Examination of other cryogens shows that only neon, having a onevatmosphere saturated liquid point on the liquidus curve which is about17 K. below its critical temperature, compares relatively closely inthis respect to helium and hydrogen. Thus, neon can be included in thegroup of low molecular weight gases (having a molecular weight belowabout 20) which can be liquefied by the process of the present inventionwith a realization of the described advantages.

It should be pointed out that in carrying out the process of theinvention using compression and expansion apparatus currently available,care should preferably be exercised to avoid compression and cooling ofthe fluids in a way such that solid material is formed (note thelocation of the saturated liquid line on the entropy temperature diagramof FIG. l). It is envisioned that in some types of heat exchangers andwork engines which may be developed in the future, the presence of solidparticles of the compressed material will not be detrimental, yand mayeven be beneficial, but such is not presently the case.

Although certain preferred embodiments of the invention have been hereindescribed in order to enable those skilled in the art to practice theprocess, it will be appreciated that various modifications and changesin the apparatus employed, and in the process parameters utilized, canbe effected without departure from the basic principles of theinvention. All changes and innovations of this type are thereforeconsidered to be encompassed by the spirit and scope of the invention.

What is claimed is:

1. A process for liquefying low molecular weight fluids wherein thefluid is helium, hydrogen, neon or mixtures thereof comprising:

compressing and cooling the fluid to a pressure above the criticalpressure and a temperature above the critical temperature so that theentropy of the compressed and cooled fluid is no greater than theentropy of the saturated liquid phase at one atmosphere pressure; thensubstantially isentropically expanding the compressed and cooled fluidthrough :an engine to accomplish work While converting the fluid to asingle phase of saturated liquid at one atmosphere pressure. 2. Aprocess as defined in claim 1 wherein said fluid is Initially compressedto a pressure exceeding the critical pressure by an amount such that thefluid can then be isobarically cooled to said temperature above thecritical temperature.

3. A process as defined in claim 2 wherein said fluid is helium and iscompressed to a pressure exceeding about 25 atmospheres beforecommencing the substantially isentropic expansion thereof.

4. A process as defined in claim 3 wherein the helium is compressed to apressure exceeding about atmospheres before commencing the substantiallyisentropic expansion.

5. A process for liquefying helium comprising: compressing helium gas toa pressure of from about to about 200 atmospheres at a temperature offrom about 65 K. to about 300 K.;

isobarically cooling the compressed helium to a temperature above thecritical temperature of helium and to an entropy value of less thanabout 0.8 calorie per gram K. without the concurrent production of solidhelium; then expanding the compressed helium to atmospheric pressurethrough a device operated by such expansion to do external work whileproducing a single liquid phase.

6. A process as defined in claim 5 wherein after compressing the heliumit is isobarically cooled to a temperature not exceeding about 9 K.

7. A process as defined in claim 5 wherein the expansion of thecompressed helium is effected substantially isentropically. v

8. A process for liquefying helium comprising:

compressing the helium to a pressure exceeding 150 atmospheres;

cooling the compressed helium isobarically to the highest temperature atwhich the cooled, compressed helium may then be expanded through anengine doing external work to yield a single saturated liquid phase atone atmosphere pressure; then expanding the cooled, compressed heliumthrough said engine to convert substantially all of the heliumto aliquid at one atmosphere pressure.

9. A process for liquefying helium comprising:

compressing the helium to a pressure above its critical pressure;

cooling the helium until its entropy is at least as low as the entropyof saturated liquid helium at one atmosphere pressure; then expandingthe helium through a Work producing engine under conditions such that nogas is produced, and the liquid produced is at atmospheric pressure.

, 8 References Cited UNITED STATES Y PATENTS 'y OTHER REFERENCES Scott,R. Bi: Cryogenic Engineering, Van Nostrand,

N.Y., 1960, pp. 67-73.

Wylen et al.: Cryogenic Engineering Fundamentals;

Dept. of Mech. Eng., Univ. of Michigan, Ann Arbor, Mich., 1962, pp. 98,100, 101.

NORMAN YUDKOFF, Primary Examiner A. F. PURCELL, Assistant Examiner Us.c1. XR.

